1. Technical Field
The present invention relates to a method for determining the amount of particulate that has accumulated in a particulate filter.
In particular, the present invention finds advantageous, though non-exclusive, application in the engine sector, particularly in internal-combustion engines of motor vehicles, and amongst these principally in diesel engines, to which the ensuing treatment will make explicit reference, without this implying any loss of generality.
Further applications of the present invention could be in fact also in fields other than the engine sector, for example, for filtering the particulate emitted by any type of system provided with particulate filters, for example, gas-oil burners for boilers, etc.
2. Description of the Related Art
As is known, diesel engines of motor vehicles emit exhaust gases that are highly pollutant, given that they contain nitrogen oxides (NOx) and particulate, i.e., unburnt particles constituted principally by carbon material. Precisely on account of the highly pollutant composition, numerous countries are introducing increasingly stringent limits to exhaust-gas emissions by internal-combustion engines in order to reduce atmospheric pollution. Consequently, the reduction of the particulate present in exhaust gases constitutes one of the targets required by future European standards on pollutant emissions (Euro5, Euro5+, and Euro6 standards).
Many are the methods up to now proposed for reducing the amount of particulate present in exhaust gases. Included amongst these methods is the use of a particulate filter, also known as particulate trap, which is generally combined to the use of an oxidizing catalytic converter, which is set upstream of the particulate filter and has the function of promoting complete oxidation of exhaust gases, converting unburnt hydrocarbons, nitrogen oxides, and carbon monoxide into dioxide carbon, water, and nitrogen. The functions of the oxidizing catalytic converter and of the particulate filter can be alternatively performed by a single component known as catalyzed particulate filter.
By way of example, represented schematically in FIG. 1, and designated as a whole by 21 is a system for discharge of the exhaust gases produced by an internal-combustion engine 22 of a motor vehicle (not illustrated), in particular a diesel engine. In the case in point, by way of non-limiting example, the internal-combustion engine 22 is of the supercharged type and comprises a turbosupercharger 23 formed by a compressor 24 set along an air-intake pipe 25 and by a turbine 26 coupled to the compressor 24 and set along an exhaust pipe 27.
The system for discharge of the gases 21 is provided with a system for post-treatment 21b of the exhaust gases, comprising a catalyzed particulate filter, i.e., comprising: an oxidizing catalytic converter 28, set along the exhaust pipe 27 in a position close to the turbosupercharger 23, and a particulate filter 29 set along the exhaust pipe 27, downstream of the oxidizing catalytic converter 28; and a further oxidizing catalytic converter 28b set along the exhaust pipe 27, upstream of the particulate filter 29.
The discharge system 21 is moreover provided with: an electronic-control system 31 comprising an airflow meter (debimeter) 32, set along the air-intake pipe 25 and generating an electrical signal indicating the flowrate of the air in the air-intake pipe 25 itself; a differential-pressure sensor 33 having a first input and a second input connected to the input and to the output, respectively, of the catalyzed particulate filter, and an output supplying an electrical signal indicating the drop in pressure across the catalyzed particulate filter; a first temperature sensor 34 set at output from the particulate filter 29 and supplying an electrical signal indicating the temperature of the exhaust gases at outlet from the particulate filter 29; a second temperature sensor 35 set on the inlet of the particulate filter 29 and supplying an electrical signal indicating the temperature of the exhaust gases at inlet of the particulate filter 29; an atmospheric-pressure sensor 36; and an electronic control unit 37 connected to the aforementioned sensors and configured for determining the amount of particulate that has accumulated in the particulate filter 29 and activating regeneration thereof upon onset of given conditions, for example, when the amount of particulate accumulated exceeds a pre-set threshold.
The particulate filter 29 has the function of mechanical barrier for the passage of the particulate and is in general constituted by parallel channels with porous walls and alternately obstructed. The obstructions force the exhaust gases to traverse the side walls of the channels so that the unburnt particles constituting the particulate are first withheld in the porosities of the side walls themselves and then, when the latter are completely filled, accumulate on the internal surfaces of the walls of the channels to form a porous layer. With the increase of the accumulation of particulate on the internal surfaces of the walls of the channels also the pressure drop on the particulate filter increases, and hence the counterpressure generated by the particulate filter itself. The particulate cannot hence be accumulated indefinitely because high accumulations cause:                deterioration of performance, drivability, and engine consumption, until even stalling of the engine may occur; and        destruction of the particulate filter, in the case of self-ignition and uncontrolled combustion of the particulate; in fact, in the presence of high accumulations of particulate, and in particular driving conditions, there may be triggered phenomena of “critical” regeneration, consisting in sudden and uncontrolled combustion of the particulate, which is in turn responsible for the high temperatures that are generated within the particulate filter and the consequent damage to the particulate filter itself.        
It is hence necessary to remove periodically the particulate trapped, performing the so-called “regeneration” of the particulate filter, i.e., proceed to removal of the particulate accumulated in the filter.
Generally, the regenerations can broadly speaking be distinguished into active regenerations, i.e., ones controlled by an electronic control unit, and spontaneous regenerations, i.e., ones triggered in an uncontrolled and unforeseeable way during a phase of accumulation, typically caused by the presence of high accumulations of nitrogen dioxide (NO2).
During operation of an internal-combustion engine, it is hence possible to distinguish phases of accumulation, i.e., time intervals in which there is a progressive accumulation of particulate in the particulate filter and there is no active regeneration, at most spontaneous regenerations, and regeneration phases, i.e., time intervals in which active regeneration occurs and the amount of particulate accumulated in the particulate filter decreases.
In the engine sector, the active regeneration of the particulate filter is obtained by means of the combustion (oxidation) of the particulate accumulated, which, since it is made up prevalently of carbon, reacts with the oxygen present in the exhaust gases, being transformed into carbon monoxide (CO) and carbon dioxide (CO2). However, this reaction occurs spontaneously only at temperatures higher than approximately 600° C., said thermal levels being much higher than those that are measured at input to the particulate filter in conditions of normal operation of the internal-combustion engine.
It is hence necessary that under certain conditions, for example when given levels of accumulation of particulate in the particulate filter are detected, the temperature of the exhaust gases at inlet to the particulate filter should be raised artificially until self-ignition of the combustion of the particulate is obtained, i.e., occurrence of the regeneration is obtained.
In internal-combustion engines provided with electronically controlled common-rail fuel-injection systems, the artificial rise in temperature of the exhaust gases is advantageously obtained by using a post injection of fuel performed in the cylinders of the internal-combustion engine following upon the main fuel injection. In particular, the post injection of fuel can be alternatively carried out either during the expansion phase, in such a way that the injected fuel will burn in the combustion chamber, thus raising the temperature of the exhaust gases produced by the internal-combustion engine, or else during the exhaust phase, in such a way that the injected fuel will not burn in the combustion chamber and will reach unburnt the oxidizing catalytic converter, thus giving rise to an exothermal reaction that raises the temperature of the exhaust gases produced by the internal-combustion engine.
Since accumulation of particulate within the particulate filter is a non-linear process, but rather depends upon the engine point, it is expedient for the regeneration to occur not periodically, for example, every 10,000 km, but according to the amount of particulate that has accumulated in the particulate filter in such a way that regeneration will occur when it is effectively necessary, with consequent optimization of the performance of the particulate filter and of the efficiency of the internal-combustion engine.
In order to determine the amount of particulate that has accumulated in the particulate filter, basically two types of models have been developed over time that are designed to estimate said amount and are to be implemented in electronic control units: statistical models and physical models.
The statistical models are based upon a collection, carried out infield and with experimental tests, of data regarding the performance of a plurality of particulate filters in a wide field of engine operating conditions, for example when it is idling, in conditions of city traffic, out-of-town traffic and motorway traffic, and in conditions of high torque and high power. The data gathered enable creation of a statistics of the accumulation of particulate within the particulate filter as the time and the engine point vary.
Said in-field collection of data makes it possible to map each individual engine point with a corresponding rate of accumulation of particulate inside the particulate filter, expressed as mass of particulate accumulated per unit time (PM[g/h]).
The amount of particulate that has accumulated in the particulate filter at a given instant is then obtained as summation of the products of the rates of accumulation for the various engine points and the time that has effectively elapsed in said engine points.
Physical models envisage, instead, calculation of the amount of particulate that has accumulated in the particulate filter on the basis of a set of data, such as the counterpressure of the particulate filter, i.e., the difference between the pressure downstream and the pressure upstream of the particulate filter, the volume flowrate of exhaust gases and the temperature of the exhaust gases.
The majority of known physical models are essentially based upon the hypotheses that the distribution of the particulate inside the channels of the particulate filter and the physico-chemical properties of the particulate itself will be uniform and constant as the engine point and the history of accumulation of the particulate vary.
For example, in Konstandopoulos A. G., Kostoglou M., Skaperdas E., Papaioannou E., Zarvalis D., and Kladopoulou E., “Fundamental Studies of diesel Particulate Filters: Transient Loading, Regeneration and Ageing”, SAE 2000-01-1016, 2000, there is proposed, on the hypotheses of a uniform spatial distribution of the particulate inside the channels (both in an axial direction and in a radial direction), an analytical modeling of the particulate filter, which takes into account different factors, such as the geometrical characteristics of the particulate filter, the physical characteristics of the material of the filter, the characteristics of the particulate itself, etc., assuming their constancy as the engine point and the history of accumulation of the particulate vary.
On the basis of the considerations set forth in the document “Fundamental Studies of Diesel Particulate Filters: Transient Loading, Regeneration and Ageing”, known in the literature is a physical model based upon the use of the following equation, which models the phenomenon of accumulation of particulate in a catalyzed particulate filter, i.e., a filter provided with a catalytic converter installed upstream of the particulate filter:
                              Δ          ⁢                                          ⁢                                    P              DPF                        ⁡                          (                                                                    Δ                    ⁢                                                                                  ⁢                                          P                      DPF                                                        2                                +                                  P                                      at                    ⁢                                                                                  ⁢                    m                                                              )                                      =                                                                              μ                  o                                ⁢                                  T                                      δ                    ⁢                                                                                  +                    1                                                  ⁢                R                                            2                ⁢                                  M                  w                                                      ⁢                          Q              m                        ⁢                                                  ⁢                                                                                (                                          H                      +                      w                                        )                                    2                                                  V                  trap                                            ·              ·                              [                                                      w                                                                  k                        m                                            ⁢                      H                                                        -                                                            1                                              4                        ⁢                                                  k                          s                                                                                      ⁢                                                                  ln                        ⁡                                                  (                                                      1                            -                                                                                                                            m                                  s                                                                                                  ρ                                  s                                                                                            ·                                                              1                                                                  NLH                                  2                                                                                                                                              )                                                                    ++                                        ⁢                                          4                      3                                        ⁢                                                                  FL                        2                                                                    H                        4                                                                              +                                                            4                      3                                        ⁢                                                                  FL                        2                                                                    H                        4                                                              ⁢                                                                  (                                                  1                          -                                                                                                                    m                                s                                                                                            ρ                                s                                                                                      ·                                                          1                                                              NLH                                2                                                                                                                                    )                                                                    -                        2                                                                                            ]                                              +                                    TR                              M                w                                      ⁢                          Q                              m                ⁢                                                                              2                        ⁢                                                  ⁢                                                                                                      (                                              H                        +                        w                                            )                                        4                                                                              H                      2                                        ⁢                                          V                      trap                      2                                                                      ⁡                                  [                                                            w                                              4                        ⁢                                                  k                          m                          ′                                                                                      +                                          2                      ⁢                                                                        ξ                          ⁡                                                      (                                                          L                              H                                                        )                                                                          2                                                                              ]                                            ++                        ⁢                                                            μ                  o                                ⁢                                  T                                      δ                    +                    1                                                  ⁢                R                                            2                ⁢                                  M                  w                                                      ⁢                          Q              m                        ⁢                                                  ⁢                                                                                (                                                                  H                        cat                                            +                                              w                        cat                                                              )                                    2                                                  V                  cat                                            ⁡                              [                                                      4                    3                                    ⁢                                                            FL                      cat                      2                                                              H                      cat                      4                                                                      ]                                                                        (        1        )            where:    Vtrap, H, L, N, w are the following geometrical properties of the particulate filter: volume; size of cells; length; number of open cells; thickness of the walls;    km, km′ are the following properties of the material of the particulate filter: linear and non-linear permeability;    Vcat, Hcat, Lcat, wcat are the following geometrical properties of the catalytic converter: volume; size of cells; length; thickness of the walls;    R, F, ξ are the following constants: gas constant (8.314 J/(K·mol)); coefficient of friction of gases in square-section pipes (˜28.454); inertial term (˜3);    Patm, Mw, T, μo are the following properties of the exhaust gases: absolute pressure downstream of the particulate filter (which can possibly be approximated with the atmospheric pressure); average molecular weight of the gas; temperature; viscosity factor;    ms, ks, ρs are the following physico-chemical properties of the particulate: mass (unknown quantity); permeability; density; and    ΔPDPF, Qm are the total drop in pressure on the particulate filter and the mass flowrate of the exhaust gases.
Implementation of the above equation at the level of the electronic control unit for determining the amount of the particulate ms that has accumulated in the particulate filter is particularly complex in so far as expressing and calculating the amount of the particulate ms as a function of the other variables requires a computational power well above that of the control units currently used in the automotive sector.
Even if said equation were implementable in the engine control unit, the results would be absolutely unsatisfactory. In fact, the present applicant has shown, with bench tests and on-vehicle tests, that both the hypotheses of uniform and constant distribution of the particulate inside the channels of the particulate filter and those of invariability of the physico-chemical properties of the particulate as the engine point and the history of accumulation of the particulate itself vary, render it impossible to perform a correct estimate of the amount of particulate accumulated in the particulate filter in real operating conditions. This is the reason why regeneration-control systems based upon the measurement of the flowrate of exhaust gases, of the temperature thereof, and of the drop in pressure of the particulate filter have never been used in the automotive sector.
An in-depth study, conducted by the present applicant in order to investigate the possibilities of definition of a modeling of the particulate filter that would be more reliable than the known ones and that would be at the same time effectively implementable on control units currently used in the automotive sector, is described in the European patent No. EP 1333165. In particular, the study conducted by the present applicant is founded upon the assumption that the hypothesis underlying the known models, whereby the distribution of the particulate inside the channels of the particulate filter and the physico-chemical properties of the particulate itself remain constant as the engine point and the history of accumulation vary, is erroneous.
Starting hence from the assumption that the distribution of the particulate inside the channels of the particulate filter and the physico-chemical properties of the particulate itself vary as the operating condition of the engine and of the history of accumulation vary, the study conducted by the present applicant has led to the definition of the equation given below that links together the drop in pressure on the particulate filter, the temperature and flowrate of the exhaust gases, and the amount of particulate that has accumulated in the particulate filter through four experimental operating parameters α, β, γ, δ:
                              Δ          ⁢                                          ⁢                                    P              DPF                        ·                          (                                                                    Δ                    ⁢                                                                                  ⁢                                          P                      DPF                                                        2                                +                                  P                                      at                    ⁢                                                                                  ⁢                    m                                                              )                                      =                                            T                              δ                +                1                                      ·                          Q              m                        ·                          (                              α                +                                  β                  ·                                      m                    S                                                              )                                +                      γ            ·            T            ·                          Q              m              2                                                          (        2        )            where:    ΔPDPF, Patm, T, Qm are respectively the drop in pressure on the particulate filter, the absolute pressure downstream of the particulate filter itself (which can be possibly approximated with the atmospheric one), the temperature and flowrate of the exhaust gases (the latter being calculable by summing the flowrate of air at inlet to the engine and the total amount of injected fuel);    mS is the amount of particulate that has accumulated in the particulate filter; and    α, β, γ, δ are the aforesaid experimental operating parameters.
The four experimental operating parameters α, β, γ, δ are determined experimentally by carrying out a specific set of bench tests with engine in steady-state running conditions. In greater detail,                α, γ depend upon the geometry of the oxidizing catalytic converter and of the particulate filter, as well as upon the properties of the material of the particulate filter itself such as porosity, pore size, etc.;        β depends upon the geometry of the particulate filter and upon the spatial distribution of the particulate inside the channels, both in an axial direction and in a radial direction, and upon the physico-chemical properties (for example, density and permeability) of the particulate; and        δ is the exponential term of the relation between the temperature of the exhaust gases and their viscosity; typically, said term is equal to 0.74.        
Given Eq. (2), it is possible obtain the amount of particulate that has accumulated in the particulate filter by applying the following equation:
                              m          s                =                                                            Δ                ⁢                                                                  ⁢                                                      P                    DPF                                    ⁡                                      (                                                                                            Δ                          ⁢                                                                                                          ⁢                                                      P                            DPF                                                                          2                                            +                                              P                                                  at                          ⁢                                                                                                          ⁢                          m                                                                                      )                                                              -                              γ                ·                T                ·                                  Q                  m                  2                                                                                    T                                  δ                  +                  1                                            ·                              Q                m                            ·              β                                -                      α            β                                              (        3        )            which can be implemented by an engine electronic control unit more easily than Eq. (1).
In detail, the operating parameter β is not kept constant, but rather it is mapped as a function of the different engine operating conditions, i.e., as a function of particular stationary conditions of accumulation of the particulate. In greater detail, initially determined and stored in the form of a map are a plurality of reference values βPDPF of a parameter β defining a relation between the amount of particulate ms that has accumulated in the particulate filter and the drop in pressure ΔPDPF on the particulate filter itself, each of the reference values βPDPF being associated to a respective steady-state condition of operation of the engine in which particulate accumulates in the particulate filter itself. In a given operating condition of the engine, there is then determined an operating value βMOD of the parameter β as a function of the reference value βPDPF of the parameter β itself for the same steady-state operating condition of the engine, and the history of accumulation of particulate in the particulate filter, i.e., the history of the engine points in which the engine itself has worked in the period that has elapsed from the last regeneration. Said operating value βMOD of the parameter β is used for calculating the amount of particulate that has accumulated in the particulate filter and triggering regeneration thereof.
Physical models enable estimation of the amount of particulate that has accumulated on the basis of known quantities, which are generally determined with the aid of sensors connected to an electronic control unit, such as air-intake flowrate sensors (debimeters), temperature sensors at input to the particulate filter, and differential pressure sensors, designed to measure the counterpressure generated by the particulate filter.
The aforementioned sensors may be subject to malfunctioning, both electrical (drifts, variations of offset and/or gain, etc.) and mechanical (clogging, accumulation of dirt, etc.), with the consequence that the measurements provided by them may be imprecise. In addition, any malfunctioning is difficult to diagnose so that it can happen that the estimation of the amount of particulate that has accumulated, in so far as it depends upon the measurements supplied by the sensors, is erroneous. In particular, in the case of overestimation, the regeneration would be activated even though there is no effective need therefor, causing a dilution of the oil for lubrication of the internal-combustion engine and a consequent risk as regards operation thereof.
In order to mitigate partially the above drawbacks, in the patent EP 1541829 filed in the name of the present applicant there is proposed a solution based upon a sort of hybrid model, implementing a threshold mechanism. According to this solution, in order to prevent possible errors in the estimates provided by a physical method from causing undue activation of regeneration, the estimates provided by the physical model are compared with a maximum value and a minimum value calculated on the basis of the estimates provided by the statistical model. In particular, the amount of particulate that has accumulated in the particulate filter calculated using the physical model is compared with a range of admissibility calculated using the statistical model. If the amount of particulate that has accumulated, calculated using the physical model, falls within the range of admissibility, then said amount of particulate that has accumulated is validated; otherwise, it is limited to the closer between the extreme values of the range of admissibility. The hybrid model guarantees that regeneration will not occur before a minimum or maximum kilometric interval has elapsed from the previous regeneration, thus preventing an excessive dilution of the lubricating oil.
However, even though they have proven to be effective in numerous practical situations, both the hybrid model and the physical model described previously present certain critical aspects in specific situations, such as turning-off of the engine, and spontaneous regenerations and deliberately interrupted active regenerations, during which they are unable to model the phenomenon of accumulation of particulate in the particulate filter in a sufficiently precise way.